Fixes to memory allocations in texture size limit testing

These patches reduce ArrayBuffer allocations in texture size limit tests, making them more feasible to pass with hardware with relatively low memory but high texture size limits, and improve readability of comments in the tests.

These patches should take care of the texture-size-limit.html test completely, which refrains from testing huge cube maps as before and covers TEXTURE_2D with narrow but tall textures which are just 1 pixel wide. However, the compressed texture test is still a problematic case as it really does need to allocate large ArrayBuffers to be able to test everything, and also uses a lot more GPU memory for texture data. There can be 2GB of texture data in a single cube map if 16384 pixels wide textures are supported, for example. Comments on whether we should reduce the test coverage on compressed cube maps to avoid running into memory limitations in the test? Otherwise the test might have too severe stability issues on many platforms to be a good inclusion in the conformance suite.